test: use bundle2 for exchange in test-push-warn

Using bundle2 for exchange unlocks the usage of changegroup version 2. This
version of the changegroup held more information (delta base) that result in
a small increase in content size (20 bytes x 2 changes).

tests/test-push-warn.t

@@ -1,3 +1,9 b''
+  $ cat << EOF >> $HGRCPATH
+  > [experimental]
+  > # drop me once bundle2 is the default,
+  > # added to get test change early.
+  > bundle2-exp = True
+  > EOF
   $ hg init a
   $ cd a
   $ echo foo > t1
@@ -150,9 +156,9 b' Specifying a revset that evaluates to nu'
   searching for changes
   2 changesets found
   uncompressed size of bundle content:
-       308 (changelog)
-       286 (manifests)
-       213  foo
+       348 (changelog)
+       326 (manifests)
+       253  foo
   adding changesets
   adding manifests
   adding file changes